Miniature microphone assemblies regularly comprise a capacitive microphone transducer electrically coupled to an integrated circuit die that comprises suitable signal amplification and conditioning circuitry. The signal amplification and conditioning circuitry may comprise a low-noise preamplifier or buffer, frequency selective filters, a DC bias voltage generator etc., adapted to amplify/buffer, filter or perform other forms of signal conditioning or generation. The integrated circuit die may comprise one or more die electrical terminal(s), for example a signal input signal terminal or a DC bias voltage terminal, electrically coupled to the capacitive microphone transducer. It is highly desirable and advantageous to provide extremely high input impedance at one or several of these die electrical terminal(s)—for example to optimize noise properties or ensure a stable DC bias voltage for the miniature microphone assembly. An extremely high input impedance at the signal input terminal ensures that loading of the capacitive microphone transducer, often having a generator impedance that corresponds to a capacitance of about 1 pF, is minimized so as to prevent attenuation of weak and fragile audio signals generated by capacitive microphone transducer in response to impinging sound.
Accordingly, this signal input terminal of the integrated circuit die is customary designed to present an input impedance higher than 100 GΩ, such as higher than 1 TΩ (1012Ω) or even several TΩ for the capacitive microphone transducer. The input impedance is often determined by an independent bias network on the integrated circuit die, for example a pair of reverse biased diodes, in combination with the previously-mentioned amplification and conditioning circuitry operatively coupled to the signal input terminal.
However, experimental work conducted by the present inventors has demonstrated the difficulty in maintaining the desired extremely high input impedance at the die electrical terminal(s) under realistic operating conditions such as, for example, environmental conditions that include exposure to moisture, cyclic heat and/or exposure to polluting agents. Under such adverse conditions, the input impedance at terminals of the integrated circuit die can be significantly degraded by a formation or absorption of a thin electrically conducting layer of moisture or water on those surfaces of the microphone carrier and/or the integrated circuit die that surround or abut the carrier electrical contact and the die electrical terminal. The formation or absorption of the thin electrically conducting layer of moisture may be caused by condensation or constant high humidity. The effect is a formation of a parallel resistive path, or current leakage path, between the die electrical terminal(s) or the carrier electrical contact and another electrical terminal of the carrier and/or integrated circuit die. The other electrical terminal may be a ground terminal or a DC voltage supply terminal. This causes a detrimental, and potentially very large, reduction of the input impedance at the die electrical terminal(s). For a signal input terminal on the integrated circuit die, the input impedance may drop from the desired range above 100 GΩ down to a range below a few GΩ, or even down to a MΩ range.
According to the present invention, the problems associated with the formation of undesired current leakage path(s) is solved by a deposition of a hydrophobic coating or layer onto the surface of the microphone carrier that holds or supports one or more high impedance carrier electrical terminals. In addition, a hydrophobic coating or layer may advantageously be deposited on surface(s) of integrated circuit die that holds high impedance electrical terminals or pads. Hydrophobic coatings or layers have been for a multitude of purposes, some of which may be seen in WO2007/112743, US2006/237806, EP1821570, WO2006/096005 and “Application of adhesives in MEMS and MOEMS assembly: a review”; Polymers and Adhesives in Microelectronics and Photonics, 2002. POLYTRONIC 2002. 2nd International IEEE Conference on Jun. 23-26, 2002, 20020623; 20020623-20020626 Piscataway, N.J., USA, IEEE, XP010594226.
Miniature microphone assemblies in accordance with the present invention are well-suited for a diverse range of applications including portable communication devices such as cellular or mobile phones, hearing aids, PDAs, game consoles, portable computers etc.